Why?

It saves having to go in to the whole explanation of what OpenID is. My new explanation goes a little like this:

You have a Flickr account, right?

Right.

Well, you just use your username on my site, Flickr will authenticate you, proving you're really who you say you are, and on you go.

Tidy.

OpenIDs can sometimes be tricky to remember/get right.

For example, Technorati's OpenID caused me no end of problems. It boiled down to the Technorati OpenID requiring 'http://' at the start - whereas I can happily use 'remysharp.com' as my OpenID - regardless of the 'http' part.

How?

The 'how' should make it even easier to get those users in to your web site. Since some of the big boys offer OpenID as part of your account, I'm going to assume that there's a large proportion of users who have an OpenID without knowing it.

Don't ask for OpenID urls: just ask for the username for X service, and construct the OpenID behind the scenes.

This example still initially points the user to an OpenID login (I'd love to get a UI designer's direction on this) - but the key effect is the ability for the user to select their login and only specify their username.

Use a cookie to store the selection of services the user chose. I don't want to have to keep selecting the right service from the drop down each time I log in.

If you can add visuals on the login: icons for AOL, Flickr, Blogger, Yahoo, etc, will immediately connect people's understanding that they're logging in to that service, not only in the input field as above, but perhaps horizontally introducing the log in.

Next time I'll show you how to glean an avatar for your user without any input from them.